Triage Action Should Use GitHub App (not a personal access token)

[jlewi]

We should modify the triage action workflow to use the kubeflow-auto-bot GitHub App rather than using a personal access token.

I think this relatively straightforward.

I think for the secret we would just store the private key for the GitHub App.

We would then just have to modify the action code to authenticate as the GitHub app using the private key

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